When creating a Department Sub-Site with Unique Permissions from the Site Creator, sometimes you might encounter the following warning message:

"Break Inheritance selected while deployment of department site, update the default groups for this modern site"

When you see this message, you need to do the following steps (manually):

1. Navigate to the newly created sub-site and go to site settings.
2. In the Site Settings, click on Site Permissions (found under Users and Permissions section).
3. You should see 3 groups created for Owners, Members and Visitors with the sub-site name.
4. Select the "Members" group, select "Settings" and in the drop down select "Make Default Group".

When we create a sub-site with unique permissions sometimes Microsoft gives an error of "Access Denied" when setting the default groups, hence we need to have this as a manual step.